If an inventor has been left out from a patent application, which has been filed, is there a deadline for including the inventor in the patent?

The situation that you deplore is not classic farmhouse anecdotal. To my knowledge and given what I have seen in relation to French, European and international (PCT) patent applications, it is not possible to return to the omission of a co-inventor in the application patent, at least once the latter published. If it is namely a Dutch patent application, perhaps the Netherlands rules permit you may remedy the situation. In all cases, the most appropriate would be that you consult a patent attorney in your country or directly the NL Patent and Trademark Office (Octrooiwet) to find out what you can do. If the administration refuses to make the addition requested, the injured co-inventor may certainly have the possibility to take legal action (action for recovery) but this action concerns normally fraudulent filings and not accidental ones...

I remain at your disposal should your inquiry relates to France or Europe.

Your sincerely,
Simon